Lesson 5 - Control Structures

Control structures allow us to conditionally execute code and create loops.

Lesson Objectives

• Conditionally execute code using if statements.
• Loop code using repeat, while, and for loops.
• Learn about the break, and next keywords.

Conditional Statements

if Statements

With conditional statements, we can test a logical expression to decide whether or not to execute lines of code. In R, this is done using an if statement.

if (logical) {
  # code
}

Take a look at the code block above. This is the general structure of an if statement. If the logical is evaluation to be TRUE, the code inside the curly brackets { } will execute. Otherwise, if the logical is evaluated to be FALSE, R will skip over all the code inside the curly brackets { }.

else Blocks

Occasionally, you’ll need to have two seperate pieces of code. One if the condition succeeds, and one if the condition fails. You can use the else keyword to do just that!

if (logical) {
  # code1
} else {
  # code2
}

In the example above, the code inside the first pair of braces { } runs if the logical evaluates to be TRUE. Otherwise, if the logical evaluates to be FALSE, the code inside the second pair of braces { } will be executed.

else if Blocks

Sometimes, if and else is not enough. You might need to test multiple ranges of numbers. This is where the elif keyword comes in.

if(logical1) {
  # code1
} else if(logical2) {
  # code2
} else {
  # code3
}

In the example above, code in # code1 is executed if logical evalutes to be TRUE. If and only if logical evaluates to be FALSE, it will test logical2. If logical2 is TRUE, the code in # code2 is executed. Otherwise, the code inside # code3 is executed.

Extra Exercises

Loops

In R , there’s often a need to repeat pieces of code multiple times, whether they’re exactly the same or with a slight variance.There are three different control structures to deal with looping, repeat loops, while loops and for loops.

repeat Loops

repeat loops are the simplest to set up, but also the simplest to mess up. Code that appears within a repeat block will keep repeating until it comes across a break keyword.

data <- c(1)

repeat {
  print(data)

  data <- c(data, length(data) + 1)

  if(length(data) == 5) {
    break
  }
}

[1] 1
[1] 1 2
[1] 1 2 3
[1] 1 2 3 4

repeat loops are not often used because while loops and for loops simplify the break condition.

If you ever find yourself using repeat loops, don’t forget the break! You may end up in an infinite loop, requiring you to force stop the code.

while Loops

The while loop is the next step up from the repeat loop. It includes the condition within the loop format, and for as long as the condition remains TRUE, the while loop will continue.

data <- c(1)

while(length(data) < 5) {
  print(data)

  data <- c(data, length(data) + 1)
}

[1] 1
[1] 1 2
[1] 1 2 3
[1] 1 2 3 4

Rather than specifying when to stop the loop, you specify a condition that must be met for the loop to continue. The condition is always checked prior to the code execution, whereas you can put the condition wherever you want inside a repeat loop. However, you can still use the break inside a while loop.

for Loops

for loops are great to iterate over items in a collection.

data <- c(1, 7, 3, 5, 8, 2, 3)
sum <- 0

for(value in data) {
  sum <- sum + value
}

print(sum)

[1] 29

In the for loop above, we iterate over every item in the vector of data we created. We then add that value to the sum variable. Once we go through all the values, we print the sum.

Another common use for the for loop is to iterate over a vector of consecutive integers using the : operator.

for(value in 1:10) {
  cat("This is loop #", value, "\n", sep="")
}

This is loop #1
This is loop #2
This is loop #3
This is loop #4
This is loop #5
This is loop #6
This is loop #7
This is loop #8
This is loop #9
This is loop #10

Key Points / Summary

• You can use if statements to create conditional code.
• Use repeat, while, and for loops to loop code or iterate through containers.